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    Thread: 1993 AAF 2.5 MT - Pinging / Spark Knocking - a new issue

    1. 04-02-2019 03:17 PM #1

      Thanks for considering my question. I have the model mentioned in the subject line. I've had it for about 6 years with pretty minimal problems. After sitting for a period of weeks, I cranked it one day. Start up was normal, but as I began to drive it seemed to spark knock or ping at even moderate throttle load. Prior to that time I only occasionally noticed pinging on a hill when I pushed or lugged the engine and had to down shift (pretty standard behavior). Now, I am puzzled as to why this pinging is so prevalent beyond light throttle. I tried some Seafoam and Techron and high octane gas, and the pinging pretty much persists. I have to be very light on the throttle to minimize it.

      Another behavior that I THINK worsened at the same time....before fully warmed up, the engine is likely to die on lift throttle/return to idle such as when pushing in the clutch as I stop for a red light. Once warmed up, it usually returns to idle just fine.

      Theories as to what may have changed while it sat?...or what I should try first to address this?

      I do all of my own wrenching and have some theories, but I am hoping to hear from some folks experienced with this engine. VCDS is not showing anything obvious to me. I have the CO potentiometer on my engine NOT the idle position switch.

      Thanks again,


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    3. 04-05-2019 09:35 AM #2
      Fuel filter was not that old or dirty, so the new one did not change anything.

      But, I am suspecting my Idle Air Control valve. It may be letting in excess air causing lean conditions. When idling after warm up and the radiator fans kick on (pretty big load), if the IAC is connected, the engine will stumble and often die. With the IAC unplugged, it hardly notices the fans coming on. I think maybe it is opening too much or sticking. I plan to clean it this weekend.

      Also, with my VCDS connected, I watched my O2 sensor / lambda voltage. It would be acting normal, oscillating about 0.8 V, but when the radiator fans come on, it would jump all over the place. Maybe a poor ground for the sensor or the fans?

      Any other thoughts?
      Last edited by scottinhim; 04-05-2019 at 09:40 AM.

    4. 05-15-2019 08:59 AM #3
      SO, I wanted to share this in case it might help someone in the future.

      Original problem was severe pinging/spark knock at all but very light throttle. The bus had sat for a while before I cranked it to renew state inspection, and that's when I noticed the pinging. I THINK the root cause or change that led to the pinging....I replaced the thermostat a couple of weeks before all of this started, and I drove it very little right after the change and parking it.

      I am guessing that the previous thermostat was keeping the engine temperature below spec, and now with the renewed standard thermostat the head is hotter and pinging resulted.

      In the process of the troubleshooting mentioned above, my coil died which threw me for a bit of a loop. But, once I figured that out and replaced it, I got back to pretty smooth operation WITH THE PINGING after the engine fully warms.

      SO, I went with the classic solution to prevent/reduce pinging, I retarded the timing. I just did this by the seat of my pants while the vehicle was idling. And, the pinging is gone, and the power seems to be about the same. Some time soon I will find and clean the timing marks on the flywheel and quantify this setting with a timing light.

      I still have some minor odd behaviors....a slight bog when I hit the throttle before the engine is warmed up, an irregular idle occasionally, a little surging when engine braking at low RPM. But, overall, it is running pretty good.


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